This invention relates to a projection tube comprising a transparent face panel being set up at a predetermined distance in front of a phosphor screen panel, the circumference of the both panels being sealed with a metal bracket and the space between the panels being filled with a transparent coolant.
Generally in the liquid cooling projection tube of this type, the heat on the phosphor screen panel is conveyed, as known, by the convection of the coolant to the peripheral frame-shaped bracket, from which the heat is dissipated. As a result, the central part of the phosphor screen panel is controlled from temperature rise, so that the luminance can be prevented from lowering and panel cracks be prevented from occurring. If such a liquid cooling means is not employed, the luminance will decrease with the operating time and can no longer be recovered. In this case, moreover, the potassium chemical tempered glass layer on the surface of the panel will be cracked because of temperature rise. The liquid cooling projection tube has been described, for example, in Japanese Patent Laid-Open No. 154146/1983 (U.S. Pat. No. 4,543,510).
The projection tube of the above type has used a coolant which is normally a liquid mixture of ethylene glycol and water, and this coolant would become colored on operation especially at a high temperature for a long time. On operation at a temperature of 60.degree. C. for 40 hours, for instance, the coolant became substantially colored, and its transmittance decreased to 60%. It goes without saying that if the temperature and/or the operating time exceeds the above, the coloring of the coolant and the drop of transmittance will further proceed. This is because the sodium in the panel glass or the potassium in the potassium chemical tempered glass layer on its surface dissolves in the water of the coolant, and these alkali metal ions act to decompose ethylene glycol, causing the coolant to decrease its transmittance and a picture defect to occur.